A vibratory power screen normally has a generally planar and longitudinally extending frame having upstream and downstream ends and a pair of laterally spaced side members extending longitudinally between the ends. A generally planar screen is spanned longitudinally within the frame normally between its ends. The entire frame is tilted down from the upstream to the downstream end. Thus when particulate material is loaded onto the screen at the upstream end the particulate material migrates down along the screen to the downstream end with the smaller fractions falling through the screen.
The screen can be vibrated as described in commonly owned U.S. Pat. No. 4,319,993 wherein drive means is provided which includes a pair of drive motors fixed to the central member of the frame and respective eccentrics carried by the motors for limitedly displacing the frame and screen relative to the support. One of the motors is mounted on the high end below the plane of the screen and the other of the motors on the low end above the plane of the screen. In this case the frame is mounted, normally by suspension from stiff hangers, for limited displacement relative to a fixed support. Thus the entire frame and screen are shaken to sieve the material.
It is also known from German patent document No. 2,133,187 to provide the frame with a plurality of longitudinally spaced and substantially parallel beater shafts underneath the screen. Respective beater arms extending generally parallel to the screen from the beater shafts have outer ends engageable with the underside of the screen. Respective actuation arms extending radially from the beater shafts have outer ends all pivoted to a stiff longitudinally extending link. Drive means on the frame connected to the link simultaneously and synchronously pivotally oscillates the beater shafts with the respective beater arms to hammer the outer ends of the beater arms against the screen. In such an arrangement the frame need not be displaceable relative to a fixed support.
The main problem with this type of machine is that the beaters do not act uniformly on the screen, which is desired to make the entire charge on the screen jump and mix, so that the finer fractions work their way down to the screen. Although the machine is set up with the beater-shaft axes all coplanar and parallel to the mesh and with the beater arms all parallel to each other, like the actuation arms, the machine quickly goes out of alignment. If the load is too heavy or the screen too loose, the screen will be lower in the middle than at the ends. Similarly the arms can deform or bearing wear can compound to produce a considerable lack of synchronization in the operation of the beaters.
Another problem with this system is that it generates considerable throw. Shock-mounting the equipment or providing counterbalances has not been effective to make the machine run smoothly and quietly, although it is universally recognized that such running of the machine will greatly prolong its life.